Aircraft control is typically achieved through the manipulation of control surfaces located on the wings and tail surfaces of the aircraft. Typical control surfaces include the elevator, rudder and ailerons. When a control surface is moved, the aerodynamic forces acting on the control surface create a reaction force that is transmitted back through the control linkages to the pilot control yoke, giving the pilot a “feel” to the control of the aircraft. A pilot can immediately feel how much control is being exerted through the control yoke before the effect is manifested in a change in aircraft attitude. The control feedback is thus a valuable aid in the flying of an aircraft. However these aerodynamic reaction forces are not constant for all flying conditions, such as airspeed and altitude. This gives rise to a residual force or back pressure that must be maintained continuously in steady level flight. In some aircraft, this force may be substantial, potentially creating significant pilot fatigue. Further, if the pilot were to release the control momentarily, under these conditions, the aircraft may quickly enter an uncomfortable or potentially dangerous attitude. To counter the residual control force, various trim mechanisms may be installed to balance the residual force for a given flight condition. A trim tab installed on the control surface is a commonly chosen solution. The trim tab may be adjusted by the pilot to neutralize back pressure for a given flight condition.
Trim is also beneficial to the operation of an autopilot. If the aircraft is kept in trim, an autopilot servo will not need to deliver as much torque, allowing lower torque limit settings and lighter servos. Further, if the aircraft is in a trimmed state, engagement and disengagement of the autopilot will be smooth. But, if the aircraft is out of trim, there will likely be a sudden pitch up or down until the pilot can recognize the condition and make corrections. Such sudden pitching can be uncomfortable or even alarming to passengers.
A trim system, like all aircraft systems, must be designed with careful consideration for safety. The potential exists for an out of control trim system to place an aircraft in a trim state where it is difficult or impossible to maintain altitude. Thus, high reliability and fail-safe design are essential elements of an aircraft trim system. Therefore, there is a need for trim control systems that automatically adjust aircraft trim and incorporate fail-safe design principles.